There are several features that help make a watch water-resistant. The most important is the gaskets, or ‘O-rings’ made of rubber, nylon or Teflon which form watertight seals at the joints where the crystal, case back and crown meet the watch case. Chronographs and other multi-function timepieces tend to have gaskets to protect the push buttons as well.

In addition, water-resistant watch cases are lined with a sealant, applied in the form of a quick-hardening liquid, which helps keep water out.

The thickness and material of the case is also a big factor in determining whether a watch can safely be worn underwater. The case must be sturdy enough to withstand pressure without caving in, which means a stainless steel or titanium case. Solid gold cases can be water resistant provided they are sufficiently thick.

A screw-in case back, as opposed to one that snaps on, also contributes to a watch’s water resistance. A screw-in crown, a feature of many divers’ watches, helps prevent water getting into the case through the spot where the watch-stem emerges. When screwed down, the crown forms a water tight seal much like screwing a lid tightly on top of a jar.

Keep in mind that there is no such thing as a water-proof watch. According to guidelines issued by the Federal Trade Commission, watch marketers are not allowed to label their watches ‘water-proof.’ Even watches designed for deep-sea diving cannot claim to be water-proof.

So how do they determine water resistance? The different levels of water resistance (often expressed in meters) are only theoretical. They refer to the depth at which a watch will keep out water if both watch and the water are perfectly motionless. These conditions, of course, are not feasible the real world. In truth, the movement of the wearer’s arm through the water increases the pressure on the watch dramatically; so it can’t be worn to the depths indicated by lab testing machines.

Also keep in mind that over time and exposure to certain chemicals or extreme temperature changes can cause the gaskets to corrode. The most common damage comes from heavily chlorinated water, spray-on colognes or hairspray (they can also damage your watch’s finish.) It’s best to have your watch’s water resistance tested every 1-2 years.